In present systems utilizing high resolution digital-to-analog converters (DAC), it is desirable in some applications to provide a ramp voltage having a variable slope and offset for a given ramp time. Such applications would be those concerned with varying a voltage that is input to a scan system with one or more axis, such as a scanning electron microscope, to move or deflect a beam.
In applications utilizing ramp generators to vary a voltage, prior systems have utilized a counter with the output thereof input to the DAC. This is utilized to provide a relatively high resolution output for control, versatility, etc. For a given ramp voltage range with a voltage that varies from a minimum to a maximum, this is a relatively straightforward process. However, when the maximum ramp voltage needs to be varied and/or the starting voltage needs to be adjusted, this becomes a more difficult problem.
One application that utilizes a varying ramp voltage is that for controlling the voltage required to move or deflect an electron beam along either the x-and/or y-axis of a scanning electron microscope (SEM). If, for example, a pattern to be scanned were defined that covered an area ten centimeters by ten centimeters, a situation may arise where only an area one centimeter by one centimeter need be scanned. Typically, the area over which the beam is scanned is defined in terms of a predetermined scan rate and a predetermined number of pixels per scan. For example, along one axis there may exist 512 pixels which would require the capture of video data 512 times during each scan of the X-axis. This would therefore require 512 pixels to be generated for a scan of the one centimeter length and 512 pixels to be generated for a scan of the ten centimeter length. This would require a voltage that would be stepped between a zero value and a first maximum corresponding to the one centimeter area, with the same voltage being stepped from a zero value to a second maximum corresponding to the ten centimeter area, that would be a factor of ten times the first maximum.
When scanning different sized areas, conventional counters present a problem in that the resolution of the counter is fixed, thus requiring a longer count to provide a higher output voltage from the DAC. Therefore, the resolution would be governed by the smallest voltage step for the smallest area to be scanned. Since each pixel for either the one centimeter area or the ten centimeter area needs to be scanned in the same amount of time, prior systems have merely increased the clock rate of the counter when scanning a ten centimeter area in order to sufficiently step the voltage for each pixel. However, this is impractical when considering a large difference in scanned areas. Furthermore, a divider circuit is needed to keep the pixel rate constant.
In view of the above disadvantages, there exists a need for a counter that operates at a given scan rate and allows a programmable number of the least significant bits to be masked off.